This invention relates to panels or wall modules of composite or sandwich construction that have good sound insulation properties while retaining the advantages of an integral construction provided by the sandwich configuration; more particularly, the invention relates to a panel having higher than mass-law transmission loss over a portion of the audio frequency band.
There are many prior-art designs for modular composite wall constructions. Four of these designs are described here. Each of these panel designs has disadvantages that affect its acoustical or structural performance, as well as its cost, and that limit its usefulness.
In these applications, transverse displacements of the face sheets are 180.degree. out of phase for symmetric motions, while for antisymmetric motions the transverse displacements of the face sheets are in phase.
(a) Styrofoam core construction: The panel consists of a fairly rigid foam such as styrofoam sandwiched between two face sheets of plywood, pressboard, gypsum board, aluminum or steel sheets or of similar materials. This construction has a severe "double wall" of symmetric mode resonance in the principal speech bands leading to a very low "STC" rating for sound insulation. Although acceptable structurally, the acoustical performance of this construction is poor. PA1 (b) Honeycomb core construction: The configuration is similar to that described above except that the core is a resin impregnated paper or aluminum honeycomb with the axis of the honeycomb cells oriented perpendicular to the face sheets. The face sheets may be made of material indicated in (a). Other materials may be used for the core. The phase speed for the antisymmetric mode is supersonic in the principal speech bands as a result of the large shear modulus of the core. The panel damping is small and insufficient to achieve a satisfactory acoustical performance. This design is similar to the "coincidence wall" design except that the phase speed for antisymmetric motions is not large enough (the shear modulus not large enough) and the panel damping is insufficient. The panel is structurally acceptable, but its acoustic performance is poor. PA1 (c) The "Soundshear" or "Shear Wall" design: The shear modulus of the core material is controlled so that the phase speed or speed of propagation for antisymmetric transverse motions of the face sheets is subsonic through the principal speech bands. The compressive stiffness of the core material perpendicular to the face sheets is large so that the double wall resonance occurs at as high a frequency as possible. PA1 (d) The "Coincidence Wall" design: The shear modulus of the core material is controlled to be large so that the antisymmetric mode of propagation is supersonic throughout the principal speech bands. The antisymmetric mode of vibration, which may be resonantly excited by incident acoustic energy, is heavily damped through the proper choice of the core material or the addition of damping layers to the core. The transmission loss (TL) for this design is strongly dependent on the panel damping. The compressive stiffness of the core is as high as possible as it is for the shear wall design.
The "shear wall" design can provide a reasonable acoustic performance for a sandwich panel design. The transmission loss is limited by limp wall mass law transmission loss in the principal speech bands. However, the core configuration required for the combination of high compressive stiffness in the thickness direction and controlled low shear modulus for shearing motions of one face sheet parallel to the other tends to be somewhat expensive to construct and requires materials that are somewhat costly by building construction standards.
The "Coincidence Wall" design requires high shear and compressive stiffness as well as high damping for the core layer(s). This design, which exists at present only in experimental configurations, has been scarcely able to reach limp wall mass law TL and would appear to be quite expensive to produce.
For typical "Shear Wall" and "Coincidence Wall" constructions, the phase speed for the antisymmetric mode of propagation in the principal speech bands is strongly dependent on the shear modulus of the core. For the "Shear Wall" the shear stiffness of the core is controlled so that the antisymmetric mode is subsonic until high frequencies above the principal speech bands. The acoustic performance is mass law limited. For the "Coincidence Wall" the shear stiffness of the core is large so that the antisymmetric mode is supersonic at low frequency. The phase speed of this mode is substantially greater than the speed of sound in the principal speech bands. The coincidence effects (tendency towards low TL) in the principal speech bands are overcome through use of a heavily damped core material or additional heavily damped core layers. The TL can, in principle, exceed mass law TL if the wall is appropriately designed to take advantage of the added damping.